Natural gas, as it occurs in nature, contains many contaminants, the most common of which is water. The dehydration of natural gas or the removal of water from natural gas is done to control the formation of gas hydrates and corrosion in distribution pipelines. The formation of gas hydrates and corrosion in distribution pipelines can actually cause the complete shutoff of gas flow under severe conditions and at best will result in decreased throughput, frozen control valves, plugged orifices and many similar operating problems.
To control the formation of gas hydrates and corrosion in natural gas distribution pipelines, the well-known method of "glycol dehydration" or "liquid desiccant dehydration" is typically used to extract water from natural gas. In conventional glycol dehydrating systems, wet natural gas enters the bottom of an absorber (or contactor) and is then contacted counter-currently with glycol or some other liquid desiccant. More specifically, the wet natural gas is mixed and/or contacted with glycol; the glycol absorbs the water from the natural gas; the glycol is then heated to separate the water from the glycol; and the substantially anhydrous glycol is recirculated back into the contactor for absorption and/or admixing with wet natural gas, in a continuous process. Suitable liquid desiccants have included ethylene glycol, diethylene glycol, and triethylene glycol. Persons skilled in the art typically show a preference for triethylene glycol.
As noted above, in conventional glycol dehydrating systems, wet natural gas is contacted with substantially anhydrous glycol in an absorber or contactor. Unfortunately, hydrogen sulfide, which is a wet natural gas impurity in some natural gas streams (termed "sour gas"), reacts with the iron in steel contactors to produce iron sulfide scale. Those skilled in the art will also understand that iron sulfide scale is also a problem in systems dehydrating sweet gas. As the concentration of iron sulfide scale increases, it plugs up the steel contactor and diminishes the contactor's capacity to remove water from wet natural gases. Eventually, the iron sulfide scale build up within the contactor becomes so large that the contactor operator must totally shut down the contactor to clean out the iron sulfide scale. Rather than incurring the expense of shutdown and cleanups, some operators have deterred the formation of iron sulfide scale by making contactors from stainless steel. But, this alternative is not free of considerable expense.
Another difficulty associated with most conventional glycol dehydration systems is that to achieve sufficient volume for effective contact between the wet natural gas and the dehydrating glycol, it has been necessary to construct contactors of relatively tall vertical height to accommodate the required number of trays. Of course, additional trays add to the cost of the contactor.
There is therefore a need in the art for an inexpensive apparatus and method for dehydrating wet natural gas which does not need to be cleaned on a frequent basis.